Abstract

The propagation of unstable wavepackets (or modes) in a one‐dimensional fluid is studied in the presence of space–time dependent fluctuations. It is shown that the fluctuations have two effects: They diffuse the modes in k space and they produce, through a short scale average, a renormalization of their growth rate. In a WKB approximation the evolution of the unstable spectrum is governed by equations which generalize the usual diffusion equation.